In a photolithography tool, a wafer stage subsystem is one of the most critical subsystems, especially in a scanner for manufacturing flat panel displays. The accuracy and stability of a wafer stage subsystem directly determines the yield, overlay accuracy and imaging quality of the photolithography tool. Compared with photolithography tools of other types, a scanner for manufacturing flat panel displays adopts a wafer stage subsystem that is typically imposed with more critical requirements for accuracy and stability because of its larger stroke, faster scanning speed, higher load, greater motor reaction and other use conditions.
With the motion precision of photolithography tools increasing, they are required to have increasingly higher anti-vibration performance. At present, active vibration dampers are employed for fine vibration isolation, which can, on the one hand, isolate important internal components from the external world including the main frame so as to create a “quiet” environment for the internal world and, on the other hand, support each of a carrier, a wafer stage, a mask stage and an objective lens. As the surface of the carrier is leveled under the control of a position control loop of the vibration dampers, and since the vibration dampers are subject to varying loads during movement of the wafer stage among various locations, in order to speed up the response of the vibration dampers, forces acting on the supported locations are calculated, with the carrier considered as a rigid body, and are applied to the control system of the vibration dampers as forward feeds. With this approach, the vibration dampers can ensure sufficient surface flatness in case of a small marble base. However, with the increasing expansion of wafer stages in size, increasingly greater sized carriers are required. When vibration dampers are deployed at edge portions of such a carrier, a central portion of the carrier may be deformed by the weight of the wafer stage and that of the carrier itself, which may in turn lead to deformation of aerostatic slideways. As a result, the aerostatic clearance may experience variations during the movement of the wafer stage, which can deteriorate the motion performance of the wafer stage. Therefore, maintaining the surface planarity of the carrier is important to the performance of the wafer stage.